Golf ball

ABSTRACT

The invention provides a golf ball having a core and ah cover of at least one layer. The core has a cross-sectional hardness which, letting R (mm) be the radius of the core, A be the JIS-C hardness at a center of the core, B be the JIS-C hardness at a position R/3 mm from the core center, C be the JIS-C hardness at a position R/1.8 mm from the core center, D be the JIS-C hardness at a position R/1.3 mm from the core center, and E be the JIS-C hardness at a surface of the core, satisfies the formulas (1) D−C≧7, (2) C−B≦7, (3) (D−C)−(C−B)≧7, and (4) E−A≧16.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of copending application Ser.No. 13/461,187 filed on May 1, 2012, the entire contents of which arehereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a golf ball having a core and a covercomposed of at least one layer. More specifically, the invention relatesto a golf ball which achieves satisfactory distances on shots with bothdrivers and middle irons used by professional golfers and skilledamateurs, and which also has an excellent durability to cracking whenrepeatedly struck.

Solid golf balls have a relatively simple inner structure composed of acore and a cover, although various golf ball properties such as rebound,feel, spin on approach shots and durability depend to a large degree onsynergy between the core and the cover. In numerous disclosures to date,efforts have been made to optimize the rebound and feel of the ball, andalso the spin rate of the ball on approach shots, by specifying indetail the cross-sectional hardness of the core. Such art is describedin, for example, the following technical literature: JP-A 2011-136020(and the corresponding published U.S. Patent Application No.2011/0159999), JP-A 2011-136021 (and the corresponding published U.S.Patent Application No. 2011/0159998), JP-A 2007-152090 (and thecorresponding U.S. Pat. No. 7,273,425), JP-A 2008-194473 (and thecorresponding U.S. Pat. No. 7,481,722), and JP-A 2010-214105 (and thecorresponding U.S. Pat. No. 7,909,710).

In addition, numerous disclosures have been made on art which, in rubbercompositions for golf ball cores, specifies the hardness profile of thecore from the standpoint of the rubber formulation. For example, JP-A2006-312044 (and the corresponding published U.S. Pat. No. 7,278,929)discloses art in which powdered sulfur is added so as to increase thehardness difference between the center and surface of the core to atleast a given value. Moreover, technical documents in which rubberformulations have been designed to achieve a desired core hardnessprofile include, for example, JP-A 2006-167452 (and the correspondingU.S. Pat. No. 7,276,560), JP-A 2006-289074 (and the corresponding U.S.Pat. No. 7,381,776), JP-A 2002-000765 (and the corresponding U.S. Pat.No. 6,679,791), JP-A 2010-188199 (and the corresponding U.S. Pat. No.6,679,791, which is the same as the preceding), JP-A 2007-167257, JP-A2008-68077 (and the corresponding U.S. Pat. No. 7,335,115), and JP-A2008-119461 (and the corresponding U.S. Pat. No. 7,300,362).

However, although a certain degree of improvement can be expected withregard to conventional rubber formulations and core hardness profiles,even further increases in distance and improvements in durability aredesired. Lately, research and development on golf balls has beenespecially intense. Improvements in the overall properties of the golfball have been desired in order to secure a competitive advantage withthe ball.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a golfball which achieves satisfactory distances both on shots with driversand on shots with middle irons used by professional golfers and skilledamateurs, and which also has an excellent durability to cracking whenrepeatedly struck.

As a result of extensive investigations aimed at achieving the aboveobjects, the inventor has developed golf ball cores havingcross-sectional hardnesses like those shown in FIGS. 3 to 7 of workingexamples of the invention; that is, cores which, in the cross-sectionalhardness profiles thereof, have a cross-sectional hardness from the corecenter to a position at a given distance therefrom that is relativelysoft and free of large fluctuations in hardness, and have across-sectional hardness thereafter up to the core surface that rises ata steep gradient. The inventor has found that golf balls having a corewith such a cross-sectional hardness achieve in particular satisfactorydistances, both on shots with drivers and on shots with middle ironssuch as a number six iron (I#6) used by professional golfers and skilledamateurs, and also exhibit an improved durability to cracking whenrepeatedly struck.

Among recent golf balls in particular, three-piece solid golf balls andfour-piece solid golf balls featuring a urethane cover are widely usedby professional golfers and skilled amateurs. The present invention, byoptimizing the internal hardness profile of the core in the above ball,improves not only the distance achieved on shots with drivers used byprofessionals and skilled amateurs, but also the distance achieved withmiddle irons such as a number six iron (I#6) used by such golfers.Moreover, the invention provides a ball which, along with improving thedistance performance, has an excellent durability to cracking whenrepeatedly struck and is capable of withstanding harsh conditions ofuse.

Accordingly, the invention provides the following golf ball.

[1] A golf ball comprising a core and a cover of at least one layer,wherein the core has a cross-sectional hardness which, letting R (mm) bea radius of the core, A be a JIS-C hardness at a center of the core, Bbe a JIS-C hardness at a position R/3 mm from the core center, C be aJIS-C hardness at a position R/1.8 mm from the core center, D be a JIS-Chardness at a position R/1.3 mm from the core center, and E be a JIS-Chardness at a surface of the core, satisfies formulas (1) to (4) below:

D−C≧7  (1)

C−B≦7  (2)

(D−C)−(C−B)≧7  (3)

E−A≧16  (4)

[2] The golf ball of claim 1, wherein the ball has a deflection, whencompressed under a final load of 5,880 N (600 kgf) from an initial loadstate of 98 N (10 kgf), of from 7 to 10 mm.[3] The golf ball of claim 1, wherein the cover comprises an outermostlayer and an intermediate layer interposed between the outermost layerand the core, and the ball has, at a surface of a sphere composed of thecore covered by the intermediate layer, a JIS-C hardness of at least 90.[4] The golf ball of claim 1, wherein the outermost layer of the covercontains a base resin comprising a polyurethane material having a ShoreD hardness of from 40 to 60.[5] The golf ball of claim 1, wherein the core has a diameter of from 32to 41 mm.[6] The golf ball of claim 1, wherein the core is composed of a singlelayer.[7] The golf ball of claim 1, wherein the difference between thecrosslink density at the core surface and the crosslink density at thecore center, both of which are crosslink densities measured based on atoluene swelling test, is at least 9×10² mol/m³.[8] The golf ball of claim 1 wherein, when the loss tangent of the corecenter is measured at a temperature of −12° C. and a frequency of 15 Hz,letting tan δ₁ be the loss tangent at a dynamic strain of 1% and tan δ₁₀be the loss tangent at a dynamic strain of 10%, the slope of these tan δvalues, expressed as [(tan δ₁₀− tan δ₁)/(10%−1%)], is not more than0.002.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 1 is a cross-sectional view showing a golf ball according to oneembodiment of the invention.

FIG. 2 is a plan view showing the dimples used in the working examplesof the invention and the comparative examples.

FIG. 3 is a graph showing the cross-sectional hardness profile of thecore used in Example 1.

FIG. 4 is a graph showing the cross-sectional hardness profile of thecore used in Example 2.

FIG. 5 is a graph showing the cross-sectional hardness profile of thecore used in Example 3.

FIG. 6 is a graph showing the cross-sectional hardness profile of thecore used in Example 4.

FIG. 7 is a graph showing the cross-sectional hardness profile of thecore used in Example 5.

FIG. 8 is a graph showing the cross-sectional hardness profile of thecore used in Comparative Example 1.

FIG. 9 is a graph showing the cross-sectional hardness profile of thecore used in Comparative Example 2.

FIG. 10 is a graph showing the cross-sectional hardness profile of thecore used in Comparative Example 3.

FIG. 11 is a graph showing the cross-sectional hardness profile of thecore used in Comparative Example 4.

FIG. 12 is a graph showing the cross-sectional hardness profile of thecore used in Comparative Example 5.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is described more fully below.

The golf ball of the invention has a construction which includes a coreand a cover of at least one layer. As mentioned above, the core may becomposed of a single layer or may be formed of a plurality of two ormore layers, although it is preferable in this invention for the core tobe composed of a single layer. Also, in this invention, the “cover”refers collectively to whatever layers are formed to the outside of thecore, and is composed of at least one layer. That is, in cases where thecover is composed of a plurality of layers, in addition to the outermostlayer of the cover, it includes also an intermediate layer interposedbetween the outermost layer and the core. Accordingly, the cover may bea two-layer cover composed of, in order from the inside: an intermediatelayer and an outermost layer. In addition, an envelope layer may beprovided between the core and the intermediate layer, in which case thecover may be a three-layer cover composed of, in order from the inside:an envelope layer, an intermediate layer and an outermost layer. A largenumber of dimples are generally formed on the outside surface of theoutermost layer of the cover.

The core used in the invention is described. This core may be obtainedby vulcanizing a rubber composition composed primarily of a rubbermaterial. No particular limitation is imposed on the rubber composition.In a preferred embodiment, the core may be formed using a rubbercomposition containing, for example, a base rubber, a co-crosslinkingagent, a crosslinking initiator, sulfur, an organosulfur compound, afiller and an antioxidant. Polybutadiene is preferably used as the baserubber of this rubber composition.

The polybutadiene serving as the above rubber component must be onehaving a cis-1,4 bond content of at least 60% (here and below, “%”refers to percent by weight), preferably at least 80%, more preferablyat least 90%, and most preferably at least 95%. If the cis-1,4 bondcontent is too low, the resilience will decrease. In addition, thepolybutadiene has a 1,2-vinyl bond content of preferably not more than2%, more preferably not more than 1.7%, and even more preferably notmore than 1.5%.

The polybutadiene has a Mooney viscosity (ML₁₊₄ (100° C.)) of preferablyat least 30, and more preferably at least 35, with the upper limit beingpreferably not more than 100, and more preferably not more than 90.

The term “Mooney viscosity” used herein refers to an industrialindicator of viscosity (JIS K6300) as measured with a Mooney viscometer,which is a type of rotary plastometer. This value is represented by theunit symbol ML₁₊₄ (100° C.), wherein “M” stands for Mooney viscosity,“L” stands for large rotor (L-type), and “1+4” stands for a pre-heatingtime of 1 minute and a rotor rotation time of 4 minutes. The “100° C.”indicates that measurement was carried out at a temperature of 100° C.

From the standpoint of obtaining a molded and vulcanized rubbercomposition having a good resilience, the polybutadiene is preferablyone synthesized with a rare-earth catalyst or a group VIII metalcompound catalyst.

Such rare-earth catalysts are not subject to any particular limitation,although preferred use may be made of a lanthanum series rare-earthcompound. Also, where necessary, an organoaluminum compound, analumoxane, a halogen-bearing compound and a Lewis base may be used incombination with a lanthanum-series rare-earth compound. Preferred usemay be made of, as the various foregoing compounds, those mentioned inJP-A 11-35633, JP-A 11-164912 and JP-A 2002-293996.

Of the above rare-earth catalysts, the use of a catalyst which employsany of the lanthanum series rare-earth elements neodymium, samarium andgadolinium is preferred, with the use of a neodymium catalyst beingespecially recommended. In such cases, a polybutadiene rubber having ahigh 1,4-cis bond content and a low 1,2-vinyl bond content can beobtained at an excellent polymerization activity.

The polybutadiene has a molecular weight distribution Mw/Mn (where “Mw”stands for weight-average molecular weight, and “Mn” stands fornumber-average molecular weight) of preferably at least 1.0, and morepreferably at least 1.3. The upper limit is preferably not more than6.0, and more preferably not more than 5.0. If Mw/Mn is too small, theworkability may decrease, whereas if it is too large, the resilience maydecline.

The above polybutadiene is used as the base rubber, in which case theproportion of the polybutadiene within the overall rubber is preferablyat least 40 wt %, more preferably at least 60 wt %, even more preferablyat least 80 wt %, and most preferably at least 90 wt %. The abovepolybutadiene may account for 100 wt %, preferably 98 wt % or less, andeven more preferably 95 wt % or less, of the base rubber.

Examples of cis-1,4-polybutadiene rubber that may be used include thehigh-cis products BR01, BR11, BR02, BR02L, BR02LL, BR730 and BR51available from JSR Corporation.

Rubber components other than the above-described polybutadiene may alsobe included in the base rubber, insofar as the objects of the inventioncan be achieved. Illustrative examples of such other rubber componentsinclude polybutadienes other than the above polybutadiene, and otherdiene rubbers, such as styrene-butadiene rubbers, natural rubbers,isoprene rubbers and ethylene-propylene-diene rubbers.

The co-crosslinking agent is not subject to any particular limitation inthis invention. Illustrative examples include unsaturated carboxylicacids, and the metal salts of unsaturated carboxylic acids. Examples ofsuitable unsaturated carboxylic acids include acrylic acid, methacrylicacid, maleic acid and fumaric acid. The use of acrylic acid ormethacrylic acid is especially preferred. The metal salts of unsaturatedcarboxylic acids are exemplified by the above unsaturated carboxylicacids which have been neutralized with a desired metal ion. Illustrativeexamples include the zinc salts and magnesium salts of methacrylic acidand acrylic acid. The use of zinc acrylate is especially preferred. Thecontent of these unsaturated carboxylic acids and/or metal salts thereofper 100 parts by weight of the base rubber is preferably at least 10parts by weight, more preferably at least 15 parts by weight, and evenmore preferably at least 20 parts by weight. The upper limit ispreferably not more than 45 parts by weight, more preferably not morethan 43 parts by weight, and even more preferably not more than 41 partsby weight.

An organic peroxide is preferably used as the crosslinking initiator.Specifically, the use of an organic peroxide having a relatively highthermal decomposition temperature is preferred. For example, an organicperoxide having an elevated one-minute half-life temperature of fromabout 165° C. to about 185° C., such as a dialkyl peroxide, may be used.Illustrative examples of dialkyl peroxides include dicumyl peroxide(“Percumyl D,” from NOF Corporation),2,5-dimethyl-2,5-di(t-butylperoxy)hexane (“Perhexa 25B,” from NOFCorporation), and di(2-t-butylperoxyisopropyl)benzene (“Perbutyl P,”from NOF Corporation). Preferred use can be made of dicumyl peroxide.These may be used singly or two or more may be used in combination. Thehalf-life is one indicator of the organic peroxide decomposition rate,and is expressed as the time required for the original organic peroxideto decompose and the active oxygen content therein to fall to one-half.The vulcanization temperature for the core-forming rubber composition isgenerally in a range of from 120 to 190° C. Within this range, thethermal decomposition of high-temperature organic peroxides having aone-minute half-life temperature of about 165° C. to about 185° C. isrelatively slow. With the rubber composition of the invention, byregulating the amount of free radicals generated, which increases as thevulcanization time elapses, a crosslinked rubber core having a specificinternal hardness profile is obtained.

The crosslinking initiator is included in an amount, per 100 parts byweight of the base rubber, of preferably at least 0.1 part by weight,more preferably at least 0.2 part by weight, and even more preferably atleast 0.3 part by weight. The upper limit is preferably not more than5.0 parts by weight, more preferably not more than 4.0 parts by weight,even more preferably not more than 3.0 parts by weight, and mostpreferably not more than 2.0 parts by weight. Including too much maymake the core too hard, resulting in an unpleasant feel at impact, andmay markedly lower the durability to cracking. On the other hand, if theamount included is too small, the core may become too soft, resulting inan unpleasant feel at impact and markedly lowering productivity.

Fillers that may be preferably used include zinc oxide, barium sulfateand calcium carbonate. These may be used singly or two or more may beused in combination. The amount of filler included per 100 parts byweight of the base rubber may be set to preferably at least 1 part byweight, and more preferably at least 3 parts by weight. The upper limitin the amount included per 100 parts by weight of the base rubber may beset to preferably not more than 200 parts by weight, more preferably notmore than 150 parts by weight, and even more preferably not more than100 parts by weight. At a filler content which is too high or too low, aproper weight and a suitable rebound may be impossible to obtain.

In the practice of the invention, an antioxidant is included in therubber composition. For example, use may be made of a commercial productsuch as Nocrac NS-6, Nocrac NS-30 or Nocrac 200 (all products of OuchiShinko Chemical Industry Co., Ltd.). These may be used singly, or two ormore may be used in combination.

The amount of antioxidant included per 100 parts by weight of the baserubber, although not subject to any particular limitation, is preferablyat least 0.05 part by weight, and more preferably at least 0.1 part byweight. The upper limit is preferably not more than 1.0 part by weight,more preferably not more than 0.7 part by weight, and even morepreferably not more than 0.4 part by weight. If the antioxidant contentis too high or too low, a suitable core hardness gradient may not beobtained, as a result of which it may not be possible to obtain a goodrebound, durability, and spin rate-lowering effect on full shots.

Sulfur may be optionally included in the rubber composition. The sulfuris exemplified by the product available from Tsurumi Chemical IndustryCo., Ltd. under the trade name “Sulfax-5.” The amount of sulfur includedcan be set to more than 0, and may be set to preferably at least 0.005part by weight, and more preferably at least 0.01 part by weight, per100 parts by weight of the base rubber. The upper limit in the amount ofsulfur, although not subject to any particular limitation, may be set topreferably not more than 0.5 part by weight, more preferably not morethan 0.4 part by weight, and even more preferably not more than 0.1 partby weight. By adding sulfur, hardness differences in the core can beincreased. However, adding too much sulfur may result in undesirableeffects during hot molding, such as explosion of the rubber composition,or may considerably lower the rebound.

In addition, an organosulfur compound may be included in the rubbercomposition so as to impart an excellent rebound. Thiophenols,thionaphthols, halogenated thiophenols, and metal salts thereof arerecommended for this purpose. Illustrative examples includepentachlorothiophenol, pentafluorothiophenol, pentabromothiophenol,p-chlorothiophenol, and the zinc salt of pentachlorothiophenol; anddiphenylpolysulfides, dibenzylpolysulfides, dibenzoylpolysulfides,dibenzothiazoylpolysulfides and dithiobenzoylpolysulfides having 2 to 4sulfurs. The use of diphenyldisulfide or the zinc salt ofpentachlorothiophenol is especially preferred.

The amount of the organosulfur compound included per 100 parts by weightof the base rubber is at least 0.05 part by weight, preferably at least0.07 part by weight, and more preferably at least 0.1 part by weight.The upper limit is not more than 5 parts by weight, preferably not morethan 4 parts by weight, more preferably not more than 3 parts by weight,and most preferably not more than 2 parts by weight. Including too muchorganosulfur compound may excessively lower the hardness, whereasincluding too little is unlikely to improve the rebound.

Decomposition of the organic peroxide within the core formulation can bepromoted by directly blending water (i.e., a water-containing material)into the above-described core material. In addition, it is known thatthe decomposition efficiency of the organic peroxide within thecore-forming rubber composition changes with temperature, and that,starting at a given temperature, the decomposition efficiency rises withincreasing temperature. If the temperature is too high, the amount ofdecomposed radicals becomes too large, leading to recombination betweenradicals and deactivation. As a result, fewer radicals act effectivelyin crosslinking. Here, when a heat of decomposition is generated bydecomposition of the organic peroxide during core vulcanization, thevicinity of the core surface remains at substantially the sametemperature as the temperature of the vulcanization mold, but thetemperature near the core center, due to the accumulation of heat ofdecomposition by the organic peroxide which has decomposed from theoutside, becomes considerably higher than the mold temperature. Whenwater (i.e., a water-containing material) is directly blended into thecore, the water acts to promote decomposition of the organic peroxide,and so a radical reaction like that described above can be made todiffer in the core center and at the core surface. That is,decomposition of the organic peroxide is further promoted near thecenter of the core, bringing about greater radical deactivation, andthus further decreasing the amount of active radicals. As a result, itis possible to obtain a core in which the crosslink densities at thecore center and at the core surface differ greatly, and also to obtain acore having different dynamic viscoelastic properties at the corecenter.

By including the above core within a golf ball, it is possible toprovide a golf ball which achieves a reduced spin rate and also has anexcellent durability and undergoes little change over time in rebound.

The water included in the core material is not particularly limited, andmay be distilled water or may be tap water. The use of distilled waterfree of impurities is especially preferred. The amount of water includedper 100 parts by weight of the base rubber is preferably at least 0.1part by weight, and more preferably at least 0.3 part by weight. Theupper limit is preferably not more than 5 parts by weight, and morepreferably not more than 4 parts by weight.

By including a suitable amount of such water, the moisture content inthe rubber composition before vulcanization becomes preferably at least1,000 ppm, and more preferably at least 1,500 ppm. The upper limit ispreferably not more than 8,500 ppm, and more preferably not more than8,000 ppm. If the moisture content of the rubber composition is toosmall, it may become difficult to obtain a suitable crosslink densityand tan δ, which may make it difficult to mold a golf ball having littleenergy loss and a reduced spin rate. On the other hand, if the moisturecontent of the rubber composition is too large, the core may become toosoft, which may make it difficult to obtain a suitable core initialvelocity.

The core can be produced by vulcanizing and curing the rubbercomposition containing the various above ingredients. For example,production may be carried out by using a mixing apparatus such as aBanbury mixer or a roll mill to mix the ingredients, carrying outcompression molding or injection molding using a core-forming mold, thensuitably heating, and thereby curing, the molded body at a temperaturesufficient for the organic peroxide and the co-crosslinking agent toact, such as from about 100° C. to about 200° C. for a period of 10 to40 minutes.

The core diameter, although not subject to any particular limitation, ispreferably at least 32 mm, and more preferably at least 33 mm. The upperlimit is preferably not more than 41 mm, and more preferably not morethan 40 mm. At a core diameter outside of this range, the ball'sdurability to cracking may dramatically decline, or the initial velocityof the ball may decrease.

It is critical for the above core to have a cross-sectional hardnesswhich, letting R (mm) be a radius of the core, A be a JIS-C hardness ata center of the core, B be a JIS-C hardness at a position R/3 mm fromthe core center, C be a JIS-C hardness at a position R/1.8 mm from thecore center, D be a JIS-C hardness at a position R/1.3 mm from the corecenter, and E be a JIS-C hardness at a surface of the core, satisfiesformulas (1) to (4) below:

D−C≧7  (1)

C−B≦7  (2)

(D−C)−(C−B)≧7  (3)

E−A≧16  (4)

By adjusting the cross-sectional hardness of the core so as to satisfyformulas (1) to (4), that is, conceptually, by finishing the core to ahardness profile in which the cross-sectional hardness from the corecenter to a position at a given distance therefrom is relatively softand free of large fluctuations in hardness and in which the subsequentcross-sectional hardness up to the core surface rises at a steepgradient, excessive deformation of the core on full shots with a driveror with a middle iron is suppressed, core deformation is optimized, anda loss of initial velocity and an increase in the spin rate can besuppressed. As a result, satisfactory distances can be obtained on shotswith drivers and on shots with middle irons such as a I#6 used byprofessional golfers and skilled amateurs, in addition to which thedurability to cracking on repeated impact can be improved.

Moreover, in above formula (1), if the D−C value is too small, asufficient spin rate-lowering effect may not be obtained, as a result ofwhich the intended distance may not be achieved. The D−C value has alower limit of preferably at least 7, and more preferably at least 8,and has an upper limit of preferably not more than 25, and morepreferably not more than 24.

Also, in above formula (2), if the C−B value is too large, a sufficientspin rate-lowering effect may not be obtained, as a result of which thedesired distance may not be achieved. The C−B value has an upper limitof preferably not more than 7, and more preferably not more than 6, anda lower limit of preferably at least −1, and more preferably at least 0.

In above formula (3), if the (D−C)−(C−B) values is too large, asufficient spin rate-lowering effect may not be obtained, as a result ofwhich the intended distance may not be obtained. The (D−C)−(C−B) valuehas a lower limit of preferably at least 7, and more preferably at least8, and has an upper limit of preferably not more than 25, and morepreferably not more than 24.

In above formula (4), the E−A value, which is the JIS-C hardnessdifference between the surface of the core and the center of the core,has a lower limit of preferably at least 16, more preferably at least17, and even more preferably at least 18, and has an upper limit ofpreferably not more than 50, more preferably not more than 45, and evenmore preferably not more than 40. If the E−A value is too large, a goodinitial velocity may not be obtained and the durability may worsen. Onthe other hand, if the E−A value is too small, the spin rate may riseexcessively, resulting in a poor distance, and the feel at impact mayharden. The JIS-C hardness at the core center, although not subject toany particular limitation, has a minimum value of preferably at least50, more preferably at least 52, and even more preferably at least 54,and a maximum value of preferably not more than 70, more preferably notmore than 68, and even more preferably not more than 66. The JIS-Chardness at the core surface, although not subject to any particularlimitation, has a minimum value of preferably at least 75, morepreferably at least 77, and even more preferably at least 79, and amaximum value of preferably not more than 100, more preferably not morethan 98, and even more preferably not more than 96.

No particular limitation is imposed on the method for adjusting thecross-sectional hardness in order to have the core satisfy aboveformulas (1) to (4), although a core having the desired cross-sectionalhardness can be obtained by suitably adjusting the core rubberformulation and the vulcanization temperature and time. For example, acore which satisfies above formulas (1) to (4) can be obtained byadjusting the type and content of the above-described organic peroxideand by adjusting the vulcanization conditions. Specifically, by usingboth an organic peroxide capable of decomposing at a high temperatureand also sulfur as rubber compounding ingredients in the core, thedesired cross-sectional hardness of this invention can easily beachieved.

Next, the crosslink density of the core is explained.

In this invention, the crosslink density at the center of the core ispreferably at least 6.0×10² mol/m³, more preferably at least 7.0×10²mol/m³, and even more preferably at least 8.0×10² mol/e. The upper limitvalue is preferably not more than 15.0×10² mol/e, more preferably notmore than 14.0×10² mol/m³, and even more preferably not more than13.0×10² mol/m³. Also, the crosslink density at the surface of the coreis preferably at least 13.0×10² mol/m³, more preferably at least14.0×10² mol/m³, and even more preferably at least 15.0×10² mol/m³. Theupper limit value is preferably not more than 30.0×10² mol/m³, morepreferably not more than 28.0×10² mol/m³, and even more preferably notmore than 26.0×10² mol/m³. The difference in crosslink density betweenthe core center and the core surface, expressed as (crosslink density ofcore surface)−(crosslink density of core center), is preferably at least9.0×10² mol/m³ and preferably not more than 30.0×10² mol/m³. If thecrosslink density at the core center or the core surface falls outsideof the above ranges, there is a possibility that the water within therubber composition may not fully contribute to decomposition of theorganic peroxide during vulcanization, as a result of which a sufficientspin rate-lowering effect on the ball may not be obtained.

The crosslink density can be measured by the following procedure.

A circular disk having a thickness of 2 mm is cut out by passing throughthe geometric center of the core. Next, 3-mm diameter samples aredie-cut from the circular disk with a die cutter at measurement placesnot more than 4 mm inside of respective sites corresponding to the corecenter and the core surface, and the sample weights are measured with anelectronic balance capable of measurement in units of two decimal places(mg). The sample and 8 mL of toluene are placed in a 10 mL vial, and thevial is closed with a stopper and left at rest for at least 72 hours,following which the solution is discarded and the sample weightfollowing immersion is measured. Using the Flory-Rehner equation, thecrosslink density of the rubber composition is calculated from thesample weights before and after swelling.

ν=−(ln(1−v _(r))+v _(r) +χv _(r) ²)/V _(S)(v _(r) ^(1/3) −v _(r)/2)

(where ν is the crosslink density, v_(r) is the volume fraction ofrubber in the swollen sample, χ is an interaction coefficient, and V_(S)is the molar volume of toluene)

v _(r) =V _(BR)/(V _(BR) +V _(T))

V _(BR)=(w _(f) −w _(f) v _(f))/ρ

V _(T)=(w _(s) −w _(f))/ρ_(T)

(where V_(BR) is the volume of butadiene rubber in the rubbercomposition, V_(T) is the volume of toluene in the swollen sample, v_(f)is the weight fraction of filler in the rubber composition, ρ is thedensity of the rubber composition, w_(f) is the sample weight beforeimmersion, w_(s) is the sample weight after immersion, and ρ_(T) is thedensity of toluene)

Calculation is carried out at a V_(S) value of 0.1063×10⁻³ m³/mol, aρ_(T) value of 0.8669, and at a value for χ, based on the literature(Macromolecules 2007, 40, 3669-3675), of 0.47.

Next, the product P×E of the difference in crosslink density P (mol/m³)between the core surface and the core center, expressed as (crosslinkdensity at core surface)−(crosslink density at core center), multipliedby the deflection E (mm) of the core when compressed under a final loadof 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) isexplained. Generally, as the core hardness becomes higher, i.e., as thecore deflection E (mm) becomes smaller, the above difference P (mol/m³)in crosslink density tends to become larger. By multiplying E by P asdescribed above, the influence of the core hardness can be canceled out,enabling the value P×E to be used as an indicator of the reduction inspin rate. The above P×E value is preferably at least 26×10² mol/m³·mm,more preferably at least 27×10² mol/m³·mm, and even preferably at least28×10² mol/m³·mm. As explained above, with the emergence of a differencein crosslink density between the core center and the core surface, agolf ball can be obtained which has a lower spin rate, a higherdurability and also, even with use over an extended period of time, nodecline in initial velocity.

Next, the method of measuring the dynamic viscoelasticity of the core isexplained.

Generally, the viscoelasticity of a rubber material is known to have astrong influence on the performance of rubber products. Also, withregard to the loss tangent (tan δ), which represents the ratio of energylost to energy stored, it is known that a smaller tan δ is associatedwith a larger contribution by the elasticity component in rubber, andthat a larger tan δ is associated with a larger contribution by theviscosity component. In this invention, in a dynamic viscoelasticitytest on vulcanized rubber at the core center in which measurement iscarried out at a temperature of −12° C. and a frequency of 15 Hz,letting tan δ₁ be the loss tangent at a dynamic strain of 1% and tan δ₁₀be the loss tangent at a dynamic strain of 10%, the slope of these tan δvalues, expressed as [(tan δ₁₀− tan δ₁)/(10%−1%)], is preferably notmore than 0.003, and more preferably not more than 0.002. If the abovetan δ values become large, the energy loss by the core becomes toolarge, which may make it difficult to obtain a satisfactory rebound anda spin rate-lowering effect. Various methods may be employed to measurethe dynamic viscoelasticity performance of the core. For example, acircular disk having a thickness of 2 mm may be cut out of the coreencased by the cover by passing through the geometric center thereof,following which, with this as the sample, a 3-mm diameter specimen maybe die-cut at the place of measurement with a die cutter. In addition,by employing a dynamic viscoelasticity measuring apparatus (such as thatavailable under the product name EPLEXOR 500N from GABO) and using acompression test holder, the tan δ values under dynamic strains of 0.01to 10% can be measured at an initial strain of 35%, a measurementtemperature of −12° C. and a frequency of 15 Hz, and the slopedetermined based on the is results of these measurements.

Of the cover used in this invention, the outermost layer material isexemplified by ionomers and polyurethanes.

From the standpoint of controllability and scuff resistance, it ispreferable to use a polyurethane as the outermost layer material. Theuse of a thermoplastic polyurethane elastomer in particular is preferredfrom the standpoint of amenability to mass production.

In cases where the outermost layer material is a thermoplasticpolyurethane elastomer, it is preferable to use one type of resin pelletcomposed of a resin blend in which the main components are (A) athermoplastic polyurethane and (B) a polyisocyanate compound and, whenthe resin pellets are charged into an injection molding machine justprior to injection molding, it is preferable for at least someisocyanate compound to be present in which all the isocyanate groups onthe molecule remain in an unreacted state. Golf balls composed of suchthermoplastic polyurethane elastomers have an excellent rebound, spinperformance and scuff resistance.

To fully and effectively achieve the objects of the invention, anecessary and sufficient amount of unreacted isocyanate groups should bepresent within the outermost layer-forming resin material. Specifically,it is recommended that the total weight of components A and B combinedbe preferably at least 60%, and more preferably at least 70%, of theoverall weight of the outermost layer. Above components A and B aredescribed in detail below.

In describing the thermoplastic polyurethane (A), the structure of thisthermoplastic polyurethane includes soft segments composed of apolymeric polyol that is a long-chain polyol (polymeric glycol), andhard segments composed of a chain extender and a polyisocyanatecompound. Here, the long-chain polyol serving as a starting material isnot subject to any particular limitation, and may be any that is used inthe prior art relating to thermoplastic polyurethanes. Exemplarylong-chain polyols include polyester polyols, polyether polyols,polycarbonate polyols, polyester polycarbonate polyols, polyolefinpolyols, conjugated diene polymer-based polyols, castor oil-basedpolyols, silicone-based polyols and vinyl polymer-based polyols. Theselong-chain polyols may be used singly or as combinations of two or morethereof. Of the long-chain polyols mentioned here, polyether polyols arepreferred because they enable the synthesis of thermoplasticpolyurethanes having a high rebound resilience and excellentlow-temperature properties.

Illustrative examples of the above polyether polyol includepoly(ethylene glycol), poly(propylene glycol), poly(tetramethyleneglycol) and poly(methyltetramethylene glycol) obtained by thering-opening polymerization of cyclic ethers. The polyether polyol maybe used singly or as a combination of two or more thereof. Of the above,poly(tetramethylene glycol) and/or poly(methyltetramethylene glycol) arepreferred.

It is preferable for these long-chain polyols to have a number-averagemolecular weight in the range of 1,500 to 5,000. By using a long-chainpolyol having such a number-average molecular weight, golf balls madewith a thermoplastic polyurethane composition having excellentproperties such as resilience and manufacturability can be reliablyobtained. The number-average molecular weight of the long-chain polyolis more preferably in the range of 1,700 to 4,000, and even morepreferably in the range of 1,900 to 3,000.

The number-average molecular weight of the long-chain polyol refers hereto the number-average molecular weight computed based on the hydroxylnumber measured in accordance with JIS K-1557.

Chain extenders that may be suitably used include those employed in theprior art relating to thermoplastic polyurethanes. For example,low-molecular-weight compounds which have a molecular weight of 400 orless and bear on the molecule two or more active hydrogen atoms capableof reacting with isocyanate groups are preferred. Examples of the chainextender include, but are not limited to, 1,4-butylene glycol,1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol and2,2-dimethyl-1,3-propanediol. Of these chain extenders, aliphatic diolshaving 2 to 12 carbons are preferred, and 1,4-butylene glycol is morepreferred.

The polyisocyanate compound is not subject to any particular limitation;preferred use may be made of one that is used in the prior art relatingto thermoplastic polyurethanes. Specific examples include one or moreselected from the group consisting of 4,4′-diphenylmethane diisocyanate,2,4-toluene diisocyanate, 2,6-toluene diisocyanate, p-phenylenediisocyanate, xylylene diisocyanate, naphthylene-1,5-diisocyanate,tetramethylxylene diisocyanate, hydrogenated xylylene diisocyanate,dicyclohexylmethane diisocyanate, tetramethylene diisocyanate,hexamethylene diisocyanate, isophorone diisocyanate, norbornenediisocyanate, trimethylhexamethylene diisocyanate and dimer aciddiisocyanate. Depending on the type of isocyanate used, the crosslinkingreaction during injection molding may be difficult to control. In thepractice of the invention, to provide a balance between stability at thetime of production and the properties that are manifested, it is mostpreferable to use 4,4′-diphenylmethane diisocyanate, which is anaromatic diisocyanate.

It is most preferable for the thermoplastic polyurethane serving asabove component A to be a thermoplastic polyurethane synthesized using apolyether polyol as the long-chain polyol, using an aliphatic diol asthe chain extender, and using an aromatic diisocyanate as thepolyisocyanate compound. It is desirable, though not essential, for thepolyether polyol to be a polytetramethylene glycol having anumber-average molecular weight of at least 1,900, for the chainextender to be 1,4-butylene glycol, and for the aromatic diisocyanate tobe 4,4′-diphenylmethane diisocyanate.

The mixing ratio of active hydrogen atoms to isocyanate groups in theabove polyurethane-forming reaction may be adjusted within a desirablerange so as to make it possible to obtain a golf ball which is composedof a thermoplastic polyurethane composition and has various improvedproperties, such as rebound, spin performance, scuff resistance andmanufacturability. Specifically, in preparing a thermoplasticpolyurethane by reacting the above long-chain polyol, polyisocyanatecompound and chain extender, it is desirable to use the respectivecomponents in proportions such that the amount of isocyanate groups onthe polyisocyanate compound per mole of active hydrogen atoms on thelong-chain polyol and the chain extender is from 0.95 to 1.05 moles.

No particular limitation is imposed on the method of preparing thethermoplastic polyurethane used as component A. Production may becarried out by either a prepolymer process or a one-shot process whichuses a long-chain polyol, a chain extender and a polyisocyanate compoundand employs a known urethane-forming reaction. Of these, a process inwhich melt polymerization is carried out in a substantially solvent-freestate is preferred. Production by continuous melt polymerization using amultiple screw extruder is especially preferred.

It is also possible to use a commercially available product as thethermoplastic polyurethane serving as component A. Illustrative examplesinclude Pandex T8295, Pandex T8290, Pandex T8283 and Pandex T8260 (allavailable from DIC Bayer Polymer, Ltd.).

Next, various types of isocyanates may be employed without particularlimitation as the polyisocyanate compound serving as component B.Illustrative examples include one or more selected from the groupconsisting of 4,4′-diphenylmethane diisocyanate, 2,4-toluenediisocyanate, 2,6-toluene diisocyanate, p-phenylene diisocyanate,xylylene diisocyanate, naphthylene-1,5-diisocyanate, tetramethylxylenediisocyanate, hydrogenated xylylene diisocyanate, dicyclohexylmethanediisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate,isophorone diisocyanate, norbornene diisocyanate, trimethylhexamethylenediisocyanate and dimer acid diisocyanate. Of the above group ofisocyanates, the use of 4,4′-diphenylmethane diisocyanate,dicyclohexylmethane diisocyanate and isophorone diisocyanate ispreferable in terms of the balance between the influence onprocessability of, e.g., the rise in viscosity accompanying the reactionwith the thermoplastic polyurethane serving as component A and thephysical properties of the outermost layer material of the resultinggolf ball.

A thermoplastic elastomer (component C) other than the above-describedthermoplastic polyurethane may be included as an optional componenttogether with components A and B. By including this component C in theabove resin blend, the flow properties of the resin blend can be furtherincreased and improvements can be made in various properties required ofthe outermost layer material of a golf ball, such as resilience andscuff resistance.

The thermoplastic elastomer other than the above thermoplasticpolyurethane which is used as component C may be of one, two or moretypes selected from among polyester elastomers, polyamide elastomers,ionomer resins, styrene block elastomers, hydrogenated styrene-butadienerubbers, styrene-ethylene/butylene-ethylene block copolymers andmodified forms thereof, ethylene-ethylene/butylene-ethylene blockcopolymers and modified forms thereof, styrene-ethylene/butylene-styreneblock copolymers and modified forms thereof, ABS resins, polyacetals,polyethylenes and nylon resins. In particular, because they increase theresilience and scuff resistance due to reaction with the isocyanategroups while at the same time maintaining a good productivity, the useof polyester elastomers, polyamide elastomers and polyacetals isespecially preferred.

The above components A, B and C have a compositional ratio, expressed asa weight ratio, which, although not subject to any particularlimitation, is preferably A:B:C=100:2 to 50:0 to 50, and more preferablyA:B:C=100:2 to 30:8 to 50.

In the present invention, the resin blend is prepared by mixing togethercomponent A, component B and, additionally, component C. At this time,it is essential to select conditions such that, of the polyisocyanatecompound, there exists at least some portion in which all the isocyanategroups remain in an unreacted state. For example, a treatment such asmixture in an inert gas such as nitrogen or In a vacuum state must beprovided. The resin blend is then injection-molded around a core thathas been placed in a mold. For easy and trouble-free handling, it ispreferable to form the resin blend into pellets having a length of 1 to10 mm and a diameter of 0.5 to 5 mm. Isocyanate groups in an unreactedstate remain within these resin pellets; while the resin blend is beinginjection-molded about the core, or due to post-treatment such asannealing thereafter, the unreacted isocyanate groups react withcomponent A and component C to form a crosslinked material.

The outermost layer may be molded by a method which involves, forexample, feeding the above-described resin blend to an injection-moldingmachine, and injecting the molten resin blend over the core. In thiscase, the molding temperature varies depending on the type ofthermoplastic polyurethane, but is preferably in the range of 150 to250° C.

When injection molding is carried out, it is desirable, though notessential, to carry out such molding in a low-humidity environment bysubjecting some or all places on the resin paths from the resin feedzone to the mold interior to purging with an inert gas such as nitrogenor a low-moisture gas such as low dew-point dry air, or to vacuumtreatment. Preferred, non-limiting, examples of the medium used fortransporting the resin under applied pressure include low-moisture gasessuch as low dew-point dry air or nitrogen. By carrying out molding insuch a low-humidity environment, reactions by the isocyanate groups arekept from proceeding before the resin is charged into the mold interior.As a result, polyisocyanate in a form where some isocyanate groups arepresent in an unreacted state is included in the molded resin material,thus making it possible to reduce variable factors such as anundesirable rise in viscosity and to increase the real crosslinkingefficiency.

Techniques that may be used to confirm the presence of polyisocyanatecompound in an unreacted state within the resin blend prior to injectionmolding about the core include those which involve extraction with asuitable solvent that selectively dissolves out only the polyisocyanatecompound. An example of a simple and convenient method is one in whichconfirmation is carried out by simultaneous thermogravimetric anddifferential thermal analysis (TG-DTA) measurement in an inertatmosphere. For example, when the resin blend (outermost layer material)which may be used in this invention is heated in a nitrogen atmosphereat a temperature ramp-up rate of 10° C./min, a gradual drop in theweight of diphenylmethane diisocyanate can be observed from about 150°C. On the other hand, in a resin sample in which the reaction betweenthe thermoplastic polyurethane material and the isocyanate mixture hasbeen carried out to completion, a weight drop is not observed from about150° C., but a weight drop can be confirmed from about 230 to 240° C.

After the resin blend has been molded as described above, the propertiesas a golf ball outermost layer can be additionally improved by carryingout annealing so as to induce the crosslinking reaction to proceedfurther. “Annealing,” as used herein, refers to aging the cover in afixed environment for a fixed length of time.

In addition to the above-described resin components, various additivesmay be optionally included in the outermost layer material in theinvention. Examples of such additives include pigments, dispersants,antioxidants, ultraviolet absorbers, ultraviolet stabilizers, partingagents, plasticizers, and inorganic fillers (e.g., zinc oxide, bariumsulfate, titanium dioxide, tungsten).

Next, the thickness of the outermost layer in this invention, althoughnot particularly limited, is preferably at least 0.1 mm, more preferablyat least 0.3 mm, and even more preferably at least 0.5 mm. The maximumthickness is preferably not more than 1.4 mm, more preferably not morethan 1.2 mm, and even more preferably not more than 1.0 mm. If theoutermost layer is thicker than the above range, the rebound on W#1shots may be inadequate or the spin rate may increase, possiblyresulting in a poor distance. If the outermost layer is thinner than theabove range, the scuff resistance may worsen, or the controllabilityeven by professional golfers and skilled amateurs may be inadequate.

The material hardness of the outermost layer, expressed as the Shore Dhardness, although not particularly limited, is preferably at least 34,more preferably at least 37, and even more preferably at least 40. Themaximum value is preferably not more than 66, more preferably not morethan 63, and even more preferably not more than 60. At a low Shore Dhardness, the ball may be too receptive to spin on full shots, possiblyresulting in a poor distance. At a Shore D hardness which is too high,the ball may not be receptive to spin on approach shots, which mayresult in a poor controllability even by professional golfers andskilled amateurs. It should be noted here that the material hardness ofthe outermost layer refers to the hardness when the material has beenmolded into a sheet of a given thickness. Below, the material hardnessesof the intermediate layer and the envelope layer are defined in the sameway.

In the golf ball of the invention, numerous dimples are provided on thesurface of the outermost layer for the sake of aerodynamic performance.The number of dimples formed on the outermost layer surface is notsubject to any particular limitation. However, to enhance theaerodynamic performance of the ball and increase the distance traveledby the ball, the number of dimples is preferably at least 250, morepreferably at least 270, even more preferably at least 290, and mostpreferably at least 300. The maximum number of dimples is preferably notmore than 400, more preferably not more than 380, and even morepreferably not more than 360.

In the practice of the invention, by further increasing the number oflayers in the ball structure, it is possible to improve the ballperformance to a level desired in particular by professional golfers andskilled amateurs. For example, an intermediate layer may be interposedbetween the above-described core and the above-described outermostlayer, although the invention is not limited to this construction.

In the above case, the material hardness of the intermediate layer,expressed as the Shore D hardness, although not particularly limited, ispreferably at least 50, more preferably at least 55, and even morepreferably at least 60. The upper limit is preferably not more than 70,more preferably not more than 66, and even more preferably not more than63. If the material hardness of the intermediate layer is too low, theball as a whole may be too receptive to spin on full shots, as a resultof which a good distance may not be achieved. On the other hand, if thematerial hardness of the intermediate layer is too high, the durabilityto cracking when repeatedly struck may worsen or the feel at impact onshots with a putter or on short approach shots may be too hard.

The thickness of the intermediate layer, although not particularlylimited, is preferably at least 0.5 mm, more preferably at least 0.7 mm,and even more preferably at least 0.9 mm. The upper limit is preferablynot more than 2.0 mm, and more preferably not more than 1.7 mm. If thethickness of the intermediate layer is larger than the above range, thespin rate-lowering effect on shots with a W#1 may be inadequate, as aresult of which a good distance may not be obtained. If the intermediatelayer is too thin, the ball may have a poor durability to cracking whenrepeatedly struck and a poor durability at low temperatures.

The intermediate layer material is not subject to any particularlimitation. For example, advantageous use may be made of known ionomerresins, thermoplastic elastomers and thermoset elastomers. Illustrativeexamples of thermoplastic elastomers include various types ofthermoplastic elastomers, such as polyester elastomers, polyamideelastomers, polyurethane elastomers, olefin elastomers and styreneelastomers. It is especially preferable to use an ionomer resin as thebase resin for the intermediate layer material. In this case, it isdesirable that formulation of the ionomer resin composition involveusing a mixture of a zinc ion (Zn²⁺)-neutralized ionomer resin and asodium ion (Na²⁺)-neutralized ionomer resin. The mixing ratiotherebetween, or zinc ion (Zn²⁺)-neutralized ionomer resin (I)/sodiumion (Na²⁺)-neutralized ionomer resin (II), expressed in terms of weightpercent, is preferably from 25/75 to 75/25, more preferably from 35/65to 65/35, and even more preferably from 45/55 to 55/45. If the (I)/(II)resin ratio does not satisfy the above range, there is a possibilitythat the rebound of the overall ball will become smaller, which may makeit impossible to obtain the desired flight performance. Moreover, thedurability to cracking when repeatedly struck at ordinary temperaturesmay worsen, and the durability to cracking at low temperatures (sub-zeroCelsius) may also worsen.

It is preferable to subject the surface of the intermediate layer toabrasion treatment so as to increase adhesion with the outermost layerlocated on the outside thereof. In addition, following such abrasiontreatment, a primer may be applied to the surface. It is also possibleto Increase adhesion by adding an adhesion reinforcing agent to theintermediate layer material.

In the practice of the invention, in addition to the above intermediatelayer, an envelope layer may be provided between the core and theintermediate layer. In such a case, there is obtained a multi-piecesolid golf ball G having a four-layer construction which, as shown inFIG. 1, is composed of, in order from the inside: a core 1, an envelopelayer 2, an intermediate layer 3, and an outermost layer 4 havingnumerous dimples D on the surface thereof.

In the above case, the envelope layer has a material hardness, expressedas the Shore D hardness, which is not particularly limited, but ispreferably at least 40, more preferably at least 42, and even morepreferably at least 44. The upper limit is preferably not more than 61,more preferably not more than 59, and even more preferably not more than57. If the material hardness of the envelope layer is too low, theoverall ball may be too receptive to spin on full shots, as a result ofwhich a good distance may not be achieved. On the other hand, if thematerial hardness of the envelope layer is too high, the durability tocracking when repeatedly struck may worsen, or the ball may have a hardfeel at impact.

The thickness of the envelope layer, although not particularly limited,is preferably at least 1.0 mm, more preferably at least 1.2 mm, and evenmore preferably at least 1.4 mm. The upper limit is preferably not morethan 4.0 mm, more preferably not more than 3.0 mm, and even morepreferably not more than 2.0 mm. At an envelope layer thickness greaterthan the above range, the initial velocity of the ball may be inadequateeven when struck with a driver, as a result of which the desireddistance may not be achieved. On the other hand, if the envelope layeris too thin, the spin rate-lowering effect may be inadequate, as aresult of which the desired distance may not be achieved.

Although no particular limitation is imposed on the envelope layermaterial, preferred use may be made of, for example, a known ionomerresin, a thermoplastic elastomer, or a thermoset elastomer. Illustrativeexamples of thermoplastic elastomers include various types ofthermoplastic elastomers, such as polyester elastomers, polyamideelastomers, polyurethane elastomers, olefin elastomers and styreneelastomers.

It is especially preferable to use, as the envelope layer material, amixture obtained by compounding 100 parts by weight of a resin component[(f)+(e)] composed of (f) a base resin of (a) an olefin-unsaturatedcarboxylic acid random copolymer and/or a metal ion neutralizationproduct of an olefin-unsaturated carboxylic acid random copolymerblended with (b) an olefin-unsaturated carboxylic acid-unsaturatedcarboxylic acid ester random terpolymer and/or a metal ionneutralization product of an olefin-unsaturated carboxylicacid-unsaturated carboxylic acid ester random terpolymer in a weightratio of from 100:0 to 0:100, and (e) a non-ionomeric thermoplasticelastomer; (c) from 5 to 80 parts by weight of a fatty acid and/or fattyacid derivative having a molecular weight of from 228 to 1500; and (d)from 0.1 to 17 parts by weight of a basic metal compound capable ofneutralizing un-neutralized acid groups in the above base resin andcomponent (c). In this case, it is preferable for the weight ratio ofthe base resin (f) to the non-ionomeric thermoplastic elastomer (e) tobe adjusted within the range of 100:0 to 100:100, and for a mixture ofcomponent (f), component (c) and component (d) to be prepared, and it isalso preferable to use all of above components (c), (d), (e) and (f). Inthis way, the ball rebound and flight performance can be furtherimproved.

In this invention, commercially available products may be used as thebase resins of components (a) and (b). Examples of commercial productswhich may be used as the random copolymer in component (a) includeNucrel 1560, Nucrel 1214 and Nucrel 1035 (all products of DuPont-MitsuiPolychemicals Co., Ltd.), and Escor 5200, Escor 5100 and Escor 5000 (allproducts of ExxonMobil Chemical). Examples of commercial products whichmay be used as the random copolymer in component (b) include NucrelAN4311, Nucrel AN4318 (both products of DuPont-Mitsui Polychemicals Co.,Ltd.), and Escor ATX325, Escor ATX320 and Escor ATX310 (all products ofExxonMobil Chemical).

Examples of commercial products which may be used as the metal ionneutralization product of the random copolymer in component (a) includeHimilan 1554, Himilan 1557, Himilan 1601, Himilan 1605, Himilan 1706 andHimilan AM7311 (all products of DuPont-Mitsui Polychemicals Co., Ltd.),Surlyn 7930 (E.I. DuPont de Nemours & Co.), and Iotek 3110 and Iotek4200 (ExxonMobil Chemical). Examples of commercial products which may beused as the metal ion neutralization product of the random copolymer incomponent (b) include Himilan 1855, Himilan 1856 and Himilan AM7316 (allproducts of DuPont-Mitsui Polychemicals Co., Ltd.), Surlyn 6320, Surlyn8320, Surlyn 9320 and Surlyn 8120 (all products of E.I. DuPont deNemours & Co.), and Iotek 7510 and Iotek 7520 (both products ofExxonMobil Chemical). Examples of zinc-neutralized ionomer resins whichare preferred as the metal ion neutralization product of the aboverandom copolymers include Himilan 1706, Himilan 1557 and Himilan AM7316.

Above component (c) is a fatty acid and/or fatty acid derivative havinga molecular weight of at least 228 but not more than 1500. Thiscomponent has a very small molecular weight compared with the baseresin. It suitably adjusts the melt viscosity of the mixture, and thushelps in particular to improve the flow properties. Component (c) ofthis invention includes a relatively high content of acid groups (orderivatives thereof), and is able to suppress an excessive loss ofresilience. Illustrative examples of the fatty acid of component (c)include myristic acid, palmitic acid, stearic acid, 12-hydroxystearicacid, behenic acid, oleic acid, linoleic acid, linolenic acid, arachidicacid and lignoceric acid. Of these, stearic acid, arachidic acid,behenic acid and lignoceric acid are preferred. Behenic acid isespecially preferred.

Preferred use may be made of a basic inorganic metal compound as thebasic metal compound of component (d). Illustrative examples of themetal ion therein include Li⁺, Na⁺, K⁺, Ca⁺⁺, Mg⁺⁺, Zn⁺⁺, Al⁺⁺⁺, Ni⁺⁺,Fe⁺⁺, Fe⁺⁺⁺, Cu⁺⁺, Mn⁺⁺, Sn⁺⁺, Pb⁺⁺ and Co⁺⁺. Known basic inorganicfillers containing these metal ions may be used as the basic inorganicmetal compound. Specific examples include magnesium oxide, magnesiumhydroxide, magnesium carbonate, zinc oxide, sodium hydroxide, sodiumcarbonate, calcium oxide, calcium hydroxide, lithium hydroxide andlithium carbonate. In particular, a hydroxide or a monoxide isrecommended. Calcium hydroxide and magnesium oxide, each of which has ahigh reactivity with the base resin, are more preferred. Calciumhydroxide is especially preferred.

Illustrative examples of component (e) include olefin elastomers,styrene elastomers, polyester elastomers, urethane elastomers andpolyimide elastomers. From the standpoint of further increasing rebound,the use of an olefin elastomer or a polyester elastomer is especiallypreferred. A commercially available product may be used as component(e). Examples include olefin elastomers such as Dynaron (JSRCorporation), and polyester elastomers such as Hytrel (DuPont-Toray Co.,Ltd.).

Various additives such as pigments, dispersants, antioxidants,ultraviolet absorbers and light stabilizers may be optionally includedin the thermoplastic resin. Specific examples of such additives includeinorganic fillers such as zinc oxide, barium sulfate and titaniumdioxide.

The above material may be obtained by mixing the various above-describedcomponents under applied heat. For example, the material may be obtainedby using a known mixing apparatus such as a kneading-type twin-screwextruder, a Banbury mixer or a kneader to knead the ingredients at aheating temperature of from 150 to 250° C. Alternatively, direct use maybe made of a commercial product, specific examples of which includethose having the trade names HPF 1000, HPF 2000 and HPF AD1027, as wellas the experimental material HPF SEP1264-3, all produced by E.I. DuPontde Nemours & Co.

The method of manufacturing multi-piece solid golf balls in which theabove-described core, envelope layer, intermediate layer and outermostlayer are each formed as successive layers is not subject to anyparticular limitation. Production may be carried out by an ordinarymethod such as a known injection molding process. By way ofillustration, first a core is placed within a given injection mold,following which the envelope layer material is injection-molded over thecore to form a first intermediate sphere, which sphere is then placed inanother injection mold and the intermediate layer material isinjection-molded over the sphere to form a second intermediate sphere.Next, this second intermediate sphere is placed in yet another injectionmold and the outermost layer material is injection-molded over thesecond intermediate sphere, concurrent with which dimples are molded inthe outermost layer surface, thereby giving a multi-piece golf ball.Alternatively, instead of the above method in which the materials forthe respective layers are injection-molded, use may made of a method inwhich each of the respective intermediate spheres is enclosed by twohalf-cups that have been molded beforehand into hemispherical shapes,and the resulting assembly is molded under applied heat and pressure.

The golf ball of the invention has a diameter of not less than 42 mm,preferably not less than 42.3 mm, and more preferably not less than 42.6mm. The upper limit in the diameter is not more than 44 mm, preferablynot more than 43.8 mm, more preferably not more than 43.5 mm, and evenmore preferably not more than 43 mm.

The weight of the golf ball is preferably not less than 44.5 g, morepreferably not less than 44.7 g, even more preferably not less than 45.1g, and most preferably not less than 45.2 g. The upper limit in theweight is not more than 47.0 g, more preferably not more than 46.5 g,and even more preferably not more than 46.0 g.

The golf ball has a deflection when subjected to a compressive load suchthat the ball deflection when compressed under a final load of 5,880 N(600 kgf) from an initial load state of 98 N (10 kgf) is preferably from7 to 10 mm, with the lower limit being preferably at least 7.1 mm, morepreferably at least 7.2 mm, and even more preferably at least 7.3 mm. Ifthe ball deflection is too small, the feel at impact may harden and thespin rate may rise excessively, as a result of which the desireddistance may not be achieved. On the other hand, if the deflection istoo large, the ball may not have a good initial velocity and thedurability may worsen.

As explained above, the golf ball of this invention providessatisfactory distances on shots with both drivers and middle irons(e.g., I#6) used by professional golfers and skilled amateurs, and alsohas an excellent durability to cracking when repeatedly struck.

EXAMPLES

Examples of the invention and Comparative Examples are given below byway of illustration, and not by way of limitation.

Examples 1 and 5, Comparative Examples 1 to 5

Golf ball cores were produced by using the rubber formulations in therespective Examples of the invention and Comparative Examples as shownin Table 1 below to prepare core compositions, then molding andvulcanizing the core compositions under the vulcanization conditions inthe table. With regard to the core in Comparative Example 5, changes inthe hardness profile were effected by impregnating the molded andvulcanized core with an acrylic acid impregnating solution.

TABLE 1 Example Comparative Example 1 2 3 4 5 1 2 3 4 5 Polybutadiene A80 80 95 100 100 50 Polybutadiene C 20 20 20 20 50 Polyisoprene rubber 5Polybutadiene B 20 20 80 80 80 80 Peroxide (1) 1.05 0.75 1.0 1.0 1.0 3.00.3 0.6 Peroxide (2) 1.2 0.3 3.0 0.6 Barium sulfate 17.8 20.9 21.5 23.421.0 Zinc oxide 27.4 19.9 4.0 4.0 4.0 4.0 20.5 10.4 7.2 5.0 Antioxidant0.2 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Zinc acrylate 36.6 33.0 37.9 35.033.5 34.0 37.5 34.0 40.0 23.5 Zinc stearate 5 5 5 5 5 Sulfur 0.085 0.060.1 0.1 Water (distilled water) 1.0 0.5 0.5 Zinc salt ofpentachlorothiophenol 0.4 0.4 0.6 0.2 0.2 0.6 1.5 1.0 1.5 0.1Vulcanization temperature (° C.) 155 155 155 155 155 155 155 160 160 150Vulcanization time (min) 21 21 13 13 13 13 15 13 13 15 Numbers in thetable indicate parts by weight.

Details on the above materials are given below.

-   Polybutadiene A: Available under the trade name “BR730” from JSR    Corporation-   Polybutadiene B: Available under the trade name “BR51” from JSR    Corporation-   Polybutadiene C: Available under the trade name “BR01” from JSR    Corporation-   Polyisoprene rubber: Available under the trade name “1R2200” from    JSR Corporation-   Peroxide (1): Dicumyl peroxide, available under the trade name    “Percumyl D” from NOF Corporation-   Peroxide (2): A mixture of 1,1-di(t-butylperoxy)-cyclohexane and    silica, available under the trade name “Perhexa C-40” from NOF    Corporation-   Antioxidant: 2,2-Methylenebis(4-methyl-6-butylphenol), available    under the trade name “Nocrac NS-6” from Ouchi Shinko Chemical    Industry Co., Ltd.-   Zinc stearate: Available under the trade name “Zinc Stearate G” from    NOF Corporation-   Sulfur: Available under the trade name “Sulfax-5” from Tsurumi    Chemical Industry Co., Ltd.-   Distilled water: Available from Wako Pure Chemical Industries, Ltd.

Next, using the respective resin materials shown in Table 2, an envelopelayer, an intermediate layer and an outermost layer were formed in thisorder over the core by injection molding, thereby forming a three-layercover. In Example 2, a two-layer cover was formed by injection-moldingan intermediate layer and an outermost layer over the core. A commondimple configuration I (338 dimples; a plan view of the dimple patternis shown in FIG. 2) was used for the dimples. The dimples were formed,during injection molding of the outermost layer, by impression withnumerous dimple-forming protrusions provided on the spherical surface ofthe mold cavity.

TABLE 2 (1) (2) (3) (4) (5) HPF 1000 100 Himilan 1605 50 Himilan 1557 15Himilan 1706 35 Nucrel AN4319 20 Nucrel AN4221C 80 Magnesium stearate 60Magnesium oxide 1.7 Trimethylolpropane 1.1 T8295 75 T8290 37.5 25 T828362.5 Titanium oxide 3.8 3.8 Polyethylene wax 1.4 1.4 Isocyanate compound7.5 7.5 Numbers in the table indicate parts by weight.

Details on the above materials are given below.

-   HPF 1000: An ionomer available from E.I. DuPont de Nemours & Co.-   Himilan: Ionomers available from DuPont-Mitsui Polychemicals Co.,    Ltd.-   Nucrel: Available from DuPont-Mitsui Polychemicals Co., Ltd.-   Magnesium oxide: Available as “Kyowamag MF150” from Kyowa Chemical    Industry Co., Ltd.-   T8925, T8290, T8283: MDI-PTMG type thermoplastic polyurethanes    available under the trade name “Pandex” from DIC Bayer Polymer-   Polyethylene wax: Available as “Sanwax 161P” from Sanyo Chemical    Industries, Ltd.-   Isocyanate compound: 4,4′-Diphenylmethane diisocyanate

Physical properties such as hardnesses of the individual layers and theball, flight performance (carry), both on shots with a W#1 and on shotswith an I#6, and durability on repeated impact were evaluated accordingto the criteria described below for the golf balls obtained in each ofExamples 1 to 5 and Comparative Examples 1 to 5. The results arepresented in Table 3 and 4. In (1) to (9) below, measurements werecarried out in a 23±1° C. environment.

(1) Golf Ball Deflection

The deflection (mm) of the golf ball when compressed at a temperature of23±1° C. and a rate of 500 mm/min under a final load of 5,880 N (600kgf) from an initial load state of 98 N (10 kgf) was measured.

(2) Center Hardness of Core

The core was cut into hemispheres and the cut face was rendered into aflat plane, following which a durometer indenter was pressedperpendicularly against the center thereof and measurement was carriedout. The JIS-C (JIS K6301-1975 standard, defined similarly below)hardness value is indicated.

(3) Surface Hardness of Core

A durometer was set perpendicular on a surface portion of the sphericalcore, and the hardness was measured based on the JIS-C hardnessstandard. The result was indicated as a JIS-C hardness value.

(4) Cross-Sectional Hardness of Core

The core was cut with a fine cutter and, letting R be the core radius(mm), B be the JIS-C hardness at a position R/3 mm from the core center,C be the JIS-C hardness at a position R/1.8 mm from the core center, andD be the JIS-C hardness at a position R/1.3 mm from the core center, theJIS-C hardness value at each of these places was measured. The corecross-sectional hardness profiles in the respective working examples andthe comparative examples are shown in the graphs in FIGS. 3 to 12.

(5) Crosslink Density of Core (Toluene Swelling Test)

The crosslink density of the core was calculated as described above inparagraph [0044].

(6) Dynamic Viscoelastic Properties of Core

The dynamic viscoelastic properties of the core were measured asdescribed above in paragraph [0046].

(7) Material Hardnesses of Envelope Layer and Intermediate Layer

The resin material for the envelope layer was formed into a sheet havinga thickness of 2 mm, and the hardness was measured with a type Ddurometer in accordance with ASTM-D2240.

(8) Surface Hardness of Intermediate Layer-Covered Sphere

The indenter of a durometer was set substantially perpendicular to thespherical surface of the intermediate layer, and the MIS-C hardness wasmeasured.

(9) Material Hardness of Outermost Layer

The measurement method was the same as in (7) above.

(10) Flight Test

The carry (m) of the ball when struck at a head speed (HS) of 50 m/swith, as the driver (W#1), a TOURSTAGE X-DRIVE 703 (loft angle, 8.5°;manufactured by Bridgestone Sports Co., Ltd.) mounted on a swing robotwas measured. The results were rated according to the criteria shownbelow. The spin rate was the value measured for the ball, immediatelyafter impact, with an apparatus for measuring initial conditions.

Good: Carry was 246 m or more

NG: Carry was less than 246 m

(11) Middle Iron (I#6)

The carry (m) of the ball when struck at a head speed of 44 m/s with, asthe middle iron, an X-BLADE CB (a number six iron manufactured byBridgestone Sports Co., Ltd.) was measured. The results were ratedaccording to the following criteria.

Good: Carry was 150 m or more

NG: Carry was less than 150 m

(12) Durability on Repeated Impact

The durability of the golf ball was evaluated using an ADC Ball CORDurability Tester produced by Automated Design Corporation (U.S.). Thistester functions so as to fire a golf ball pneumatically and cause it torepeatedly strike two metal plates arranged in parallel. The incidentvelocity against the metal plates was set at 43 m/s. The number of shotsrequired for the golf ball to crack was measured. The results were ratedaccording to the following criteria.

-   -   Good: Number of shots until cracking occurred was 150 or more    -   NG: Number of shots until cracking occurred was less than 150

TABLE 3 Example Comparative Example 1 2 3 4 5 1 2 3 4 5 Core Diameter(mm) 35.4 37.7 36.3 36.3 36.3 35.4 35.4 35.4 35.4 35.4 A (core center)61 64 63 63 59 60 59 64 59 61 B (R/3 mm from core 61 65 68 67 68 69 7673 69 66 center) C (R/1.8 mm from core 66 67 69 68 69 69 76 74 72 70center) D (R/1.3 mm from core 80 76 79 79 79 73 76 77 71 76 center) E(core surface) 90 82 90 90 84 80 84 84 89 81 D − C 14 9 10 11 10 4 0 3−1 6 C − B 5 2 1 1 1 0 1 1 3 4 (D − C) − (C − B) 9 7 9 10 8 4 −1 2 −4 2E − A 29 17 27 27 25 20 25 19 30 20 Crosslink Core 11 11 11 11 11 10 1111 11 11 density center (×10² Core 24 22 24 24 23 17 21 20 24 18 mol/m³)outside Core 13 11 13 13 12 7 10 9 13 7 outside − Center Tan δ at 1%Strain 0.0470 0.0498 0.0487 0.0486 0.0449 0.0460 0.0448 0.0501 0.04480.0467 core 10% Strain 0.0597 0.0655 0.0591 0.0613 0.0593 0.0823 0.06930.0892 0.0718 0.0834 center Slope of 0.0014 0.0017 0.0012 0.0014 0.00160.0040 0.0027 0.0043 0.0030 0.0041 10% strain and 1% strain CoverEnvelope Material (1) (1) (1) (1) (2) (1) (1) (1) (1) layer Thickness1.7 1.3 1.3 1.3 1.7 1.7 1.7 1.7 1.7 (mm) Shore D 51 51 51 51 55 51 51 5151 hardness Inter- Material (3) (3) (3) (3) (3) (3) (3) (3) (3) (3)mediate Thickness 1.1 1.7 1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1 layer (mm)Shore D 62 62 62 62 62 62 62 62 62 62 hardness JIS-C 98 98 98 98 98 9898 98 98 98 hardness at surface of inter- mediate layer OutermostMaterial (4) (5) (5) (5) (5) (4) (4) (4) (4) (4) layer Thickness 0.8 0.80.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 (mm) Shore D 44 47 47 47 44 44 44 44 4444 hardness Ball Deflection (mm) 8.2 8.8 9.1 9.2 9.7 8.2 8.4 8.4 8.8 8.9under 600 kg load

TABLE 4 Example Comparative Example 1 2 3 4 5 1 2 3 4 5 Flight W#1,Initial 73 73 73 73 73 74 74 74 73 73 HS, velocity 50 m/s (m/s) Spinrate 2,455 2,383 2,357 2,377 2,327 2,507 2,478 2,488 2,453 2,413 (rpm)Carry (m) 247 246 248 248 247 246 245 245 245 246 Rating good good goodgood good good NG NG NG good I#6 Initial 55 55 54 54 54 54 54 54 54 54velocity (m/s) Spin rate 6,279 5,851 6,120 6,236 6,136 6,735 6,623 6,6616,525 6,205 (rpm) Carry (m) 150 152 152 152 153 148 149 149 149 151Rating good good good good good NG NG NG NG good Durability on good goodgood good good good NG good good NG repeated impact (43 m/s)

As shown in Table 4 above, the golf balls obtained in the examples ofthe invention had an excellent flight performance when struck using aW#1 and an I#6, and also had a good durability. By contrast, the ballsobtained in Comparative Examples 1 to 5 either had a poor flightperformance when a W#1 or an I#6 was used, or had a poor durability.

1. A golf ball comprising a core and a cover of at least one layer,wherein the core has a cross-sectional hardness which, letting R (mm) bea radius of the core, A be a JIS-C hardness at a center of the core, Bbe a JIS-C hardness at a position R/3 mm from the core center, C be aJIS-C hardness at a position R/1.8 mm from the core center, D be a JIS-Chardness at a position R/1.3 mm from the core center, and E be a JIS-Chardness at a surface of the core, satisfies formulas (1) to (4) below:D−C≧7  (1)C−B≦7  (2)(D−C)−(C−B)≧7  (3)E−A≧16  (4)
 2. The golf ball of claim 1, wherein the ball has adeflection, when compressed under a final load of 5,880 N (600 kgf) froman initial load state of 98 N (10 kgf), of from 7 to 10 mm.
 3. The golfball of claim 1, wherein the cover comprises an outermost layer and anintermediate layer interposed between the outermost layer and the core,and the ball has, at a surface of a sphere composed of the core coveredby the intermediate layer, a JIS-C hardness of at least
 90. 4. The golfball of claim 1, wherein the outermost layer of the cover contains abase resin comprising a polyurethane material having a Shore D hardnessof from 40 to
 60. 5. The golf ball of claim 1, wherein the core has adiameter of from 32 to 41 mm.
 6. The golf ball of claim 1, wherein thecore is composed of a single layer.
 7. The golf ball of claim 1, whereinthe difference between the crosslink density at the core surface and thecrosslink density at the core center, both of which are crosslinkdensities measured based on a toluene swelling test, is at least 9×10²mol/m³.
 8. The golf ball of claim 1 wherein, when the loss tangent ofthe core center is measured at a temperature of −12° C. and a frequencyof 15 Hz, letting tan δ₁ be the loss tangent at a dynamic strain of 1%and tan δ₁₀ be the loss tangent at a dynamic strain of 10%, the slope ofthese tan δ values, expressed as [(tan δ₁₀−tan δ₁)/(10%−1%)], is notmore than 0.002.